1. Field of The Invention
The present invention relates to methods for determining the causes of human ocular diseases. In particular, the invention relates to anatomical, histochemical and molecular biological methods for evaluating the effects of elevated intraocular pressure on various tissues in mammalian eyes. The invention specifically relates to glaucoma in humans and experimentally-induced glaucoma in laboratory rats. The invention also provides methods for evaluating the efficacy of treatment strategies for glaucoma, especially those treatment strategies involving glaucoma-relieving drugs.
2. Background of the Invention
Glaucoma is a major cause of blindness, affecting nearly 2% of the adult population in the United States. In glaucoma, optic nerve fibers connecting the retina to the brain are damaged at the level of the optic nerve head, giving it a characteristic appearance termed glaucomatous optic neuropathy. Although many factors influence this process, elevated intraocular pressure (IOP; normally equal to 5-25 mm of Hg) is most well documented, and attempts to reduce elevated IOP form the basis for all current glaucoma therapy. Most instances of elevated intraocular pressure are due to increased resistance to aqueous humor outflow from the trabecular meshwork, a sieve-like ringed structure located at the juncture of the iris and cornea.
Conventional glaucoma therapy concentrates on lowering IOP either through eye drops, laser treatments, or surgery, all of which have significant drawbacks. Eye drops have been prescribed (either alone or usually in combination) either to inhibit the secretion of aqueous humor by the ciliary processes "beta-blockers") or to improve escape of aqueous humor from the eye (miotics such as pilocarpine). To be effective, however, these drugs need to be administered in highly concentrated dosages because of poor penetration of the drugs into the eye. The administration of these drugs in such highly concentrated dosages creates a strong potential for ocular and systemic side effects, since a large percentage of the amount of these drugs administered topically to the eye(s) drain into the nose and are absorbed into the bloodstream through the nasal lining. This is particularly a problem when the drugs must be administered on a continuous basis over a patient's lifetime.
Laser treatment is often used as an alternative or in addition to drug treatment, but the results are variable and often transient. The best current surgical treatment involves creating a channel for the fluid to escape out of the eye, thereby lowering the intraocular pressure. Although effective in controlling the eye pressure, this surgery has significant potential for producing sight-threatening complications including infection and bleeding, especially immediately after surgery (when eye pressure suddenly and dramatically decreases).
All of these therapies carry risks, ranging from the systemic side effects of ocular medications to surgical complications such as abrupt, catastrophic loss of vision. Moreover, the precise benefits of lowering eye pressure in all glaucoma patients have not been unequivocally demonstrated. Furthermore, the relation of other factors to the glaucomatous process and how such factors may affect optic nerve susceptibility to IOP is poorly understood. Better understanding of these issues will provide a more rational approach to glaucoma therapy. In addition, such understanding will lead to new treatments for glaucoma that "protect" the optic nerve head in the face of elevated as well as normal intraocular pressure.
Since most forms of glaucoma are associated with elevated IOP, nearly all current glaucoma therapies are designed to lower eye pressure. Elevated IOP is the most common and best understood factor influencing the development of glaucomatous optic neuropathy (see Armaly, 1980, Survey Ophthalmol. 21: 139-144). In patients, elevation of IOP secondary to trauma or inflammation produces characteristic glaucomatous optic neuropathy, and nearly all humans will develop such changes if the IOP is elevated high enough on a chronic basis (Van Buskirk & Cioffi, 1992, Am. J. Ophthalmol. 113: 447-452). Experimental elevation of IOP in laboratory animals mimics human glaucoma, with characteristic optic nerve cupping, obstructed axoplasmic flow and preferential loss of large axons (see de Kater et al., 1986, Invest. Ophthalmol. Vis. Sci. 27: 1751-1754; Bunt-Milam et al., 1987, Exp. Eye Res. 44: 537-551; Morrison et al., 1990, Arch. Ophthalmol. 108: 1020-1024; Knepper et al., 1991, Exp. Eye Res. 52: 525-533).
However, many other factors are known or suspected to influence this process, including vascular disease, anatomy, age and prior optic nerve damage. Many patients with characteristic glaucomatous optic neuropathy do not have elevated IOP (Gliklich et al., 1989, Ophthalmol. 96: 316-320). This suggests that factors other than pressure may influence the glaucomatous process, or may make some individuals more susceptible to developing glaucoma, even though they have a "normal" IOP (see Cartwright et al., 1986, Arch. Ophthalmol. 106: 989-900). Other patients suffer from episodic vascular disturbances such as vasospasm and migraine headaches. There is some evidence that the visual field in these patients may be improved with peripheral vasodilator drugs. Although optic nerve head and retinal blood flow is efficiently autoregulated in normal animals to an IOP within 25 mm Hg of the mean arterial pressure (Drance et al., 1988, Am. J. Ophthalmol. 105: 35-39), non-invasive studies in normal humans suggest that autoregulation deteriorates when IOP rises above 27 mm Hg (Riva et al., 1986, Invest. Ophthalmol. Vis. Sci. 27: 1707-1712). Diabetes has also been associated with glaucomatous optic neuropathy. In light of these clinical observations, optic nerve head susceptibility may be linked to a defect in vascular autoregulation, either pre-existing or wherein such autoregulation deteriorates when the IOP rises above 27 mm Hg (Riva et al., ibid.).
Age and optic nerve head structure appear to be other factors involved in the development of glaucomatous optic neuropathy. Increases in susceptibility of the superior and inferior regions of the optic nerve head appear to correlate with regional variations in the structure of the lamina cribrosa, whereby the laminar beams are more sparse and thin in these regions, thereby providing less support for axon bundles (see Quigley et al., 1983, Am. J. Ophthalmol. 95: 673-691). Glaucomatous optic neuropathy is also more common in myopic patients whose discs are often larger than normal (Wilson et al., 1987, Arch. Ophthalmol. 105: 1066-1071; Tuulonen & Airaksinen, 1992, Arch. Ophthalmol. 110:211-213). Despite the fact that average intraocular pressures in black and white individuals is about the same, the incidence of glaucomatous optic neuropathy in blacks is nearly five times that in whites, suggesting that the optic nerve head is more susceptible to whatever etiological agent is ultimately responsible for glaucomatous damage in blacks (Sommer et al., 1991, Arch. Ophthalmol. 109: 1090). There is in addition a correlation between the incidence of glaucomatous optic neuropathy and a greater average optic disc size in the black population.
Prior optic nerve damage also appears to play a role in the development of glaucomatous optic neuropathy, correlating with the clinical impression that nerves already damaged by glaucoma are abnormally vulnerable to further elevations in IOP (Drance et al., ibid.). Also, because the total number of axons in the optic nerve gradually decrease with age (Morrison et al., 1990, Invest. Ophthalomol. Vis. Sci. 31: 1623), age frequently appears to be a contributing risk factor for development of glaucomatous optic neuropathy.
It is evident that the interrelationship of all of these factors must be thoroughly understood in order to develop a rational approach to glaucoma therapy that eliminates optic nerve damage and minimizes needless therapeutic risks and side effects.
Currently, the direct benefits of lowering IOP are poorly understood in many patients. This is primarily due to the lack of an inexpensive, well characterized animal model in which the detailed cellular responses of the optic nerve to elevated IOP can be studied. In addition to accurately representing many secondary forms of human glaucoma, such a model would provide crucial information on the cell biology of pressure-induced optic nerve damage. Better understanding of the cellular effects of elevated pressure on the optic nerve would improve understanding of the potential benefits of lowering eye pressure in glaucoma patients, regardless of the mechanism of optic tissue damage. In addition, increased knowledge about the events surrounding the development of pressure-induced optic neuropathy will better enable the study of human glaucoma and improve ways of evaluating the relative contributions of intraocular pressure and other potential disease-promoting factors.
Until now, most experimental animal models of glaucomatous optic neuropathy have monitored either the presumed initial event in optic nerve damage (i.e., obstruction of axoplasmic flow) or the chronic pathology of long term damage, such as histologic changes in optic nerve head structure. Since it is likely that cellular changes in optic nerve fibers and their associated glial tissues begin shortly after elevation of IOP (and before gross histologic evidence of damage becomes apparent) and persist throughout the disease process, sensitive indicators of optic nerve damage should be discovered by searching for and detecting these subtle cellular processes. These indicators may then be used to evaluate the importance of intraocular pressure as well as other factors in the glaucomatous process.
The use of monkeys of various species in experimentally-induced animal models of human glaucoma is known in the art.
Pasquale et al., 1992, Opthalmol. 99: 14-18 disclose the use of monkeys for evaluating the efficacy of mitomycin C treatment therapy following full sclerostomy to relieve experimentally-induced elevated intraocular pressure.
Jampel et al., 1991, Arch. Ophthalmol. 108: 430-435 relates to the efficacy of bioerodable polyanhydride discs containing 5-fluorouridine following filtration surgery on glaucomatous monkeys.
Alvarado, 1990, Trans. Am. Ophthalmol. Soc. 87: 489-514 disclose the use of liposome-encapsulated 5-fluoroorotate to promote post-surgical wound healing following glaucoma surgery on monkeys.
Lee et al., 1988, Invest. Ophthalmol. Vis. Sci. 29: 1692-1697 disclose the efficacy of bioerodable polyanhydride discs containing 5-fluorouridine following filtration surgery on glaucomatous monkeys.
Lee et al., 1985, Curr. Eye Res. 4: 775-781 relates to pharmacological testing of putative intraocular pressure lowering drugs in a laser-induced monkey glaucoma model.
Iwata et al., 1985, Graefes. Arch. Clin. Exp. Ophthalomol. 223: 184-189 disclose defects in retinal nerve fibres associated with argon laser-induced glaucoma in cynomolgus monkeys.
Pederson & Gaasterland, 1984, Arch. Ophthalmol. 102: 1689-1692 disclose the development of glaucoma in monkey eyes treated with light from an argon laser.
Gressell et al., 1984, Ophthalomology 91: 378-383 disclose the use of 5-fluorouracil to inhibit scar tissue formation at the site of glaucoma surgery performed on owl monkey eyes.
Quigley & Hohman, 1983, Invest. Ophthalmol. Vis. Sci. 24: 1305-1307 disclose the development of glaucoma in monkey eyes treated with light from an argon laser.
Similarly, the use of rabbits in experimentally-induced animal models of human glaucoma is known in the art.
Finger et al., 1991, Arch. Ophthalmol. 109: 1001-1004 disclose the use of microwave thermotherapy in the treatment of experimentally-induced glaucoma in rabbit eyes.
Lu et al., 1990, J. Ocul. Pharmacol. 6: 271-278 disclose the systemic and topical use of 6-hydroxyethoxy-2-benzothiazole sulfonamide to relieve elevated intraocular pressure in .alpha.-chymotrypsin-induced glaucoma in rabbit eyes.
Miller et al., 1990, Ophthalmic Surg. 21: 44-54 relates to the use of topical dexamethasone and .beta.-irradiation in conjunction with fistulizing surgery to relieve experimentally-induced glaucoma in rabbit eyes.
Miller et al., 1989, Ophthalmic Surg. 20: 350-357 relates to a model for glaucoma fistulizing surgery in rabbits.
Bunt-Milam et at., 1987, Exp. Eye Res. 44: 537-551 disclose changes in optic nerve head axonal transport in rabbits having hereditary glaucoma.
Gherezghiher et al., 1986, Exp. Eye Res. 43: 885-894 relates to laser-induced glaucoma in rabbits as an animal model for primary human glaucoma.
Miller et al., 1985, Trans. Ophthalmol. Soc. UK 104: 893-897 describe a rabbit model for glaucoma fistulizing surgery.
Anderman et al., 1982, J. Ft. Ophthalmol. 5: 499-504 relates to the use of .alpha.-chymotrypsin-induced glaucoma in rabbit eyes for the evaluation of intraocular pressure-lowering drugs.
Rowland et al., 1981, Curr. Eye Res. 1: 169-173 disclose a circadian rhythm in intraocular pressure in rabbits.
Light-induce glaucoma in birds has also been studied.
Lauber, 1991, J. Ocul. Pharmacol. 7: 65-75 disclose the use of light-induced avian glaucoma to test the efficacy of anti-myopic drugs.
Lauber, 1987, J. Ocul. Pharmacol. 3: 77-100 provides a review of light-induced avian glaucoma as an animal model for primary human glaucoma.
de Kater et al., 1986, Invest. Ophthalmol. Vis. Sci. 27: 1751-1754 relates to the use of the Slate turkey, which suffers a hereditary eye disease leading to secondary angle closure glaucoma as an animal model for human glaucoma.
Takatsuji et al., 1986, Invest. Ophthalmol. Vis. Sci. 27: 396-400 relates to the use of albino mutant quails as an animal model for human glaucoma.
Lauber et al., 1985, Can. J. Ophthalmol. 20: 147-152 disclose the use of light-induced avian glaucoma to evaluate the intraocular pressure-lowering effects of timolol and pilocarpane.
A variety of other animals have been used to investigate the causes of glaucoma and the efficacy of putative glaucoma treatment strategies.
Yan et al., 1991, Invest. Ophthalmol. Vis. Sci. 32: 2515-2520 relates to the use of enucleated pig eyes in vitro for investigating the role of hydrogen peroxide insult in the development of primary open-angle glaucoma.
Baranov et al., 1991, Vestn. Ofthalmol. 107: 9-14 relates to the use of surgical intervention to relieve experimentally-induced glaucoma in rats.
Svee & Strosberg, 1986, Invest. Ophthalmol. Vis. Sci. 27: 401-405 disclose the therapeutic use and systemic side effects of ocular .beta.-adrenergic antagonists in anesthetized dogs.